1. Field of the Invention
The present invention relates to a photoelectric conversion apparatus of the type that accumulates carriers generated by photoexcitation.
2. Related Background Art
FIG. 6A is a schematic cross-sectional view of a photoelectric conversion apparatus disclosed in the Japanese Laid-open Patent Gazettes Nos. 12759-12765/1985, and FIG. 6B is an equivalent circuit diagram of a photoelectric conversion cell thereof.
As shown in these drawings, photoelectric conversion cells are arranged on an n-type silicon substrate 1, and each cell is electrically insulated from neighboring cells by separating areas 2 consisting of SiO.sub.2, Si.sub.3 N.sub.4 or polysilicon.
Each photoelectric conversion cell is constructed in the following manner.
On an n.sup.- -area 3 of a low impurity concentration formed for example by epitaxial technology, a p-area 4 is formed by doping of p-type impurity, and in said p-area 4 there is formed an n.sup.+ -area 5 is formed by impurity diffusion or ion implantation. The p-area 4 and n.sup.+ -area 5 constitute the base and emitter of a bipolar transistor.
An oxide film 6 is formed on the n.sup.- -area 3 having the above-mentioned areas, and a capacitor electrode 7 of a predetermined area is formed on said oxide film 6. The capacitor electrode 7 is positioned opposite to the p-type base area 4 across the oxide film 6, and is given a pulse voltage to control the potential of the floating p-base area 4.
An emitter electrode 8 is connected to the n.sup.+ -emitter area 5. There are also formed, on the bottom face of the substrate 1, an n.sup.+ -area 11 with a high impurity concentration, and a collector electrode 12 for giving a potential to the collector of the bipolar transistor.
The basic function of the above-explained apparatus is as follows. In an initial state the p-type base area 4 of the bipolar transistor assumes a negative potential. Light 13 is introduced to said p-type base area 4 to generate electron-hole pairs, of which positive holes are accumulated in said p-type base area 4 to elevate the potential thereof in the positive direction (accumulating operation).
Then a positive voltage pulse for signal reading is given to the capacitor electrode 7, and a read-out signal corresponding to the change in the base potential in the accumulating operation is released from the floating emitter electrode 8 (signal reading operation). This reading operation can be repeated, since the amount of accumulated charge in the p-type base area 4 scarcely changes.
For eliminating the positive holes accumulated in the p-type base area 4, the emitter electrode 8 is grounded and a positive refreshing pulse is supplied to the capacitor electrode 7. Said pulse biases the p-type base area 4 in the forward direction with respect to the n.sup.+ -type emitter area 5, thereby allowing elimination of the accumulated positive holes. The p-type base area 4 returns to the initial state at the end of said refreshing pulse (refreshing operation). The operations of accumulation, reading and refreshing can thereafter be repeated in the same manner.
In summary, the above-explained process accumulates the photogenerated carriers in the p-type base area 4 and controls the current between the emitter electrode 8 and the collector electrode 12 by the amount of accumulated charge. Thus the amount of accumulated carriers is read after amplification by the amplifying function of each cell, so that the device can provide a high output, a high sensitivity and a low noise level.
In this process the potential Vp generated in the base by the photoinduced carriers (positive holes in this case) accumulated therein is given by Q/C, wherein Q is the amount of charge of the positive holes accumulated in the base area and C is the capacitance connected to the base area. As will be apparent from this relation, at a higher degree of integration, the Q and C decrease together with the reduction in cell size, so that the potential Vp generated by photoexcitation is maintained almost constant. Consequently this proposed process is advantageous also for the future development toward a higher degree of resolution.
However, in the conventional photoelectric conversion apparatus explained above, in which the refreshing operation for dissipating the carriers accumulated in the base area relies on the forward current between the emitter and the base, the base potential after the refreshing operation is inevitably related to that before the refreshing if the refreshing pulse is short, thus giving rise to drawbacks of a retentive image and a non-linearity in the photoelectric conversion characteristic.
FIG. 6C is a schematic plan view of another photoelectric conversion apparatus described in the above-mentioned patent references, and FIGS. 6D and 6E are respectively a cross-sectional view along a line A--A' and an equivalent circuit diagram.
As shown in these drawings, photoelectric conversion cells are arranged on an n-type silicon substrate 601, and each cell is electrically insulated from neighboring cells by separating areas 602 consisting of SiO.sub.2, Si.sub.3 N.sub.4 or polysilicon.
Each cell is constructed in the following manner.
On an n.sup.- -type area 603 of a low impurity concentration formed for example by an epitaxial process, a p-type base area 604 and a p-type area 605 are formed by doping a p-type impurity such as boron, and an n.sup.+ -type emitter area 606 is formed in the p-type base area 604.
Said p-type base area 604 and the p-type area 605 constitute source and drain areas of a p-channel MOS transistor to be explained later.
An oxide film 607 is formed on the n.sup.- -type area 603 containing the above-mentioned areas, and a gate electrode 608 for said MOS transistor and a capacitor electrode 609 are formed on said oxide film 607. The capacitor electrode 609 is positioned opposite to the p-type base area 604 across the oxide film 607, thus constituting a capacitor for controlling the base potential.
There are also provided an emitter electrode 610 connected to the n.sup.+ -type emitter area 606 and an electrode 611 connected to the p-type area 605, and, on the bottom face of the substrate 601, a collector electrode 612 across an ohmic contact layer.
In the following there will be explained the function of the above-explained photoelectric conversion cell.
Light is introduced into the p-type base area 604 to accumulate the carriers (positive holes in this case) corresponding to the amount of incident light in the p-type base area 604 (accumulating operation).
The accumulated carriers causes a change in the base potential, which is read from the emitter electrode 610. Thus an electric signal can be obtained corresponding to the amount of incident light (reading operation).
In the following there will be explained a refreshing operation for dissipating the positive holes accumulated in the p-type base area 604.
FIGS. 6F and 6G are wave form charts showing the refreshing operation.
As shown in FIG. 6F, the MOS transistor is turned on when a negative voltage exceeding a threshold value is applied to the gate electrode 608.
For refreshing, as shown in FIG. 6G, the emitter electrode 610 is grounded, and the electrode 611 is also brought to the ground potential. Then a negative voltage is applied to the gate electrode 608 to turn on the p-channel MOS transistor, whereby the p-type base area 604 is brought to a constant potential regardless of the accumulated potential. Then a refreshing positive pulse is applied to the capacitor electrode 609 to bias the p-type base area 604 in the forward direction with respect to the n.sup.+ -type emitter area 606, whereby the accumulated positive holes are eliminated through the grounded emitter electrode 610. The p-type base area 604 returns to the initial state of negative potential at the end of the refreshing pulse (refreshing operation).
In this manner the remaining charge is dissipated by the refreshing pulse after the potential of the p-type base area 604 is brought to a constant value by the MOS transistor, so that the new accumulating operation can be conducted independently from the preceding accumulating operation. Also a high speed operation is possible since the retentive charge can be rapidly dissipated.
It is also possible to complete the refreshing operation by turning on the MOS transistor, through the application of a voltage for complete refreshing to the electrode 611.
The operations of accumulation, reading and refreshing can thereafter be repeated in the same manner.
However such conventional photoelectric conversion apparatus is still associated with a drawback of a low aperture ratio, due to the presence of the refreshing MOS transistor on the light-receiving surface, and the presence of wirings required for supplying a constant voltage to the electrode 611 and for supplying pulses to the gate of the refreshing MOS transistor, particularly in case of forming an area sensor. Also the structure is complicated by the increased number of driving pulses.
Also in such conventional apparatus, the readout signal contains unnecessary signals such as driving noises and dark signals.
The driving noises are generated in the read-out of the signal by driving the photosensor, and include noises resulting from fluctuations in the manufacture, for example in the shape of each element, and smears resulting from the separation of elements and depending on the amount of incident light.
The dark signals are caused by dark current in the photosensor, and fluctuate mainly depending on the accumulating time of the photosensor and the circumferential temperature.
Such driving noises and dark signals become a problem particularly in image taking under a low luminosity, since, in such condition, the obtained signal level is inevitably low, so that such unnecessary signals reduce the S/N ratio and deteriorate the image quality. Such unnecessary signals have to be reduced in order to improve the image quality.
However, as explained above, the dark signals depend significantly on the temperature and the accumulating time, while the driving noises are much less dependent on such factors. Consequently the elimination of such unnecessary signals require the separation of these signals and determination of independent correction coefficients, for which a large memory capacity is required, further leading to a complex signal processing, an increased cost and a large space requirement for the device.
In order to avoid such drawbacks, the present applicant already proposed, in the Japanese Patent Application No. 229625/1986, a photoelectric conversion device capable of providing an output signal of a high S/N ratio with a simple structure, by subtracting, from the read-out signal of a photoelectric conversion element, the retentive signal after refreshing thereof, thereby eliminating the unnecessary components such as dark signals and driving noises.
However such structure is still unable to avoid the loss in the aperture ratio, caused by the presence of the separating areas and the refreshing transistors.